1. Field of the Invention
The present invention relates to a separating device in an apparatus for forming an image using an electrophotographic system, for example, an electrophotographic copying machine, a laser beam printer and the like.
2. Description of the Prior Art
An electrophotographic copying machine, for example, is provided with a photosensitive drum. At the time of copying, the surface of the photosensitive drum is charged to a predetermined potential. The charged surface of the photosensitive drum is exposed to light corresponding to a document image. Consequently, charged charges are selectively eliminated from the surface of the photosensitive drum, so that an electrostatic latent image of the document image is formed. The electrostatic latent image is then developed using toner. Specifically, toner is applied to the surface of the photosensitive drum from which the charged charges are selectively eliminated by the exposure, so that the toner is electrostatically adsorbed only on a portion where the charged charges remain.
Thereafter, recording paper adheres to the photosensitive drum, so that transferring discharge is applied to the reverse surface of the paper (the surface opposite to the surface adhering to the photosensitive drum). For example, when the photosensitive drum using selenium for a photosensitive layer, the photosensitive drum is first positively charged. On the other hand, the toner is negatively charged, so that it is electrostatically adsorbed on positive charges which exist on the surface of the photosensitive drum. In this case, positive DC discharge is used as the transferring discharge. By this transferring discharge, strong positive charges are applied to the paper, so that the toner negatively charged which is electrostatically adsorbed on the positive charges on the photosensitive drum is electrostatically attracted by the positive charges on the paper.
After transfer, separating discharge is applied to the reverse surface of the paper. The separating discharge is induced so as to discharge the respective charges on the photosensitive drum, the paper and the toner and separate from the photosensitive drum the paper and the toner which are electrostatically adsorbed on the surface of the photo-sensitive drum. AC discharge having an offset value set to the minus side is generally used as the separating discharge. The positive charges on the paper are discharged by the separating discharge.
Meanwhile, in a separating corona discharger for inducing the separating discharge, various devices have been conventionally considered so as to reliably separate the paper from the photosensitive drum.
For example, Japanese Patent Laid-Open Gazette Nos. 115068/1980 and 115069/1980 have proposed a separating device in which a separating corona discharger is provided with two discharge wires and the respective discharge wires produce the effects of discharging which differ in magnitude, and a separating device in which at least two separating corona dischargers are provided and the respective dischargers produce the effects of discharging which differ in magnitude.
Furthermore, Japanese Utility Model Laid-Open Gazette No. 194759/1984 discloses a separating device in which a separating corona discharger is provided with two discharge wires, and the two discharge wires are made equal to each other in wire diameter and are made different from each other in height from the bottom surface of a shielding case to make discharge characteristics to paper by the discharge wires different from each other.
Furthermore, Japanese Patent Laid-Open Gazette No. 56978/1987 discloses a separating device in which a plurality of discharge electrodes having an equal shape are arranged in a separating corona discharger, and the respective electrodes are arranged at an equal distance from the surface of a photosensitive drum.
Various improvements have been thus conventionally proposed with respect to the separating devices using discharge. The reason for this is that the frequency of occurrence of inferior separation in separating the paper from the photosensitive drum is high due to, for example, the change in the use conditions, the environment or the like.
On the other hand, if paper is subjected to too excessive discharging by the separating device, a toner image once transferred to the paper is transferred again to the photosensitive drum, which is not preferable, as also described in the foregoing Japanese Patent Laid-Open Gazette No. 56978/1987. Alternatively, the toner image transferred to the paper is distorted even if it is not transferred again.
In the separating device, therefore, devices must be considered so as to discharge charged on the paper by transferring discharge, to satisfactorily separate the paper and the toner from the photosensitive drum and as not to distort the toner image transferred to the paper. In considering the devices, it is necessary to meet such conflicting conditions that the toner image is distorted if the separating performance is thought too important, while the separating performance is lowered if the image quality is thought important.
In the conventional separating devices, various devices are considered so as to meet the above described conflicting conditions. Examples are such a device that a so-called double wire structure in which two discharge wires are provided is used and the respective discharge wires are made different from each other in height from the bottom surface of a shielding case to make discharge characteristics by the discharge wires different from each other (for example, a separating device described in Japanese Utility Model Laid-Open Gazette No. 194759/1984), and such a device that separate power supplies are respectively connected to two discharge wires and different discharge voltages are applied to the discharge wires (for example, a separating device described in Japanese Patent Laid-Open Gazette No. 115068/1980).
In the separating device so constructed that the two discharge wires are made different from each other in height from the bottom surface of the shielding case, however, it is difficult to adjust the positional relationship between the path for paper and the separating device. The reason for this is that the distance between the discharge wire whose height is made large and the photosensitive drum is naturally decreased, so that the wire is easily brought into contact with the paper. In addition, if the height of the discharge wire is made large, shielding effects by the shielding case are degraded, so that stable discharge is not easily obtained and interference is liable to occur between separating discharge and transferring discharge. Furthermore, the ratio of minus and plus components cannot be altered for each wire.
On the other hand, the separating device so constructed that the discharge voltages are respectively supplied to the two discharge wires from the separate power supplies has the disadvantage in that two power supplies are required, thereby to complicate the separating device and increase the manufacturing cost.